As is known, it is necessary to vary the distribution of brake forces to the front wheels and the rear wheels in order to compensate for static and dynamic axle load shifts. A great variety of mechanical, invariably adjusted pressure reducing valves or load-responsive and/or deceleration-responsive brake force regulators are used for this purpose. However, only a relatively coarse adaptation to the actual axle load distribution may be achieved by the use of these mechanical devices.
German Patent No. 33 23 402 discloses electronic brake force distributors which are equipped with electrically operable hydraulic valves serving to control the pressure variation in the wheel brakes. In the simplest case, the front-wheel brakes are connected directly to the braking pressure generator of the brake system, while the hydraulic conduit leads to the rear-wheel brakes through hydraulic valves. The rotational behavior of the wheels is measured by wheel sensors, and the hydraulic valves are actuated such that the brake slip of the rear wheels remains within a defined relationship to the brake slip of the front wheels. It is preferred in the above referenced patent that the brake slip of the rear wheels is limited by the electronic brake force distribution control to a value of 85 to 97% of the front-wheel brake slip.
An electronic control of this type is advisable in particular in connection with an anti-lock system because the hydraulic valves required for the anti-lock control may be used without modification for the electronic control of brake force distribution as well. The same applies to the wheel sensors. The result is that only minimal extra effort and equipment is required to extend the anti-lock control system to a system with electronic control of the brake force distribution.
However, the disadvantage of controlling the brake force distribution in the described way, i.e. by using as the control quantity the difference between the brake slip of the front axle and the brake slip of the rear axle, is that pressure limitation on the rear wheels commences too early in the event of either brake circuit failure at the front axle or a major brake pressure imbalance between the front-wheel brakes. This disadvantage occurs in particular when both front-wheel brakes are connected to the same pressure fluid circuit, that means in a so-called black and white brake circuit split-up.
Further, German Patent No. 34 18 044 discloses a multiple-circuit hydraulic brake system including wheel sensors, hydraulic valves in the brake lines and a monitoring circuit which, by means of the sensors, determines the reaction of the wheels to brake application and evaluates it for error detection.